This invention relates to a method of growing high quality, nonpolar Group III nitrides. Group III nitrides that are conventionally grown on c-plane sapphire substrates have a c-plane surface, which is inherently polar due to the crystallographic location of the Group III atoms and nitrogen atoms in the (0001) plane (c-plane). The resulting internal electric fields are advantageous for applications where the formation of a two-dimensional electron gas is desired, as in transistors. However, the fields cause spatial separation of electrons and holes within quantum wells, which produces both a red-shift of optical transitions and a reduction of the oscillator strength. Elimination of such polarization-induced field effects could be achieve by the use of nonpolar nitride surfaces, such as the a-plane surface. In the absence of polarization along the growth direction, undesirable shifting and reduction of emission intensity can be achieved; flat-band conditions can also be achieved without the application of an external electric field.
Nonpolar (11 20) a-plane GaN has been grown on (1 102) r-plane sapphire substrates, with [0001]GaN∥ [ 1101]sapphire and [ 1100]GaN∥ [11 20]sapphire (M. D. Craven, S. H. Lim, F. Wu, J. S. Speck, and S. P. DenBaars, “Nonpolar (11 20) a-plane Gallium nitride Thin films Grown on (1 102) r-plane Sapphire: Heteroepitaxy and Lateral Overgrowth,” Phys. Stat. Sol. (a), vol. 194 (2002). pp. 541-544). Threading dislocation (TD) densities of approximately 2.6×1010/cm−2 were reported for heteroepitaxial continuous a-plane GaN. Reduction of threading dislocations was obtained by lateral epitaxial overgrowth over a SiO2 mask with mask stripe openings aligned along [ 1100]GaN.
Cuomo and coworkers report a method utilizing sputter transport techniques to produce arrays or layers of self-forming, self-oriented columnar structures characterized as discrete, single-crystal Group III nitride posts or columns on various substrates. The columnar structure is formed in a single growth step. A group III metal source vapor is produced by sputtering a target, for combination with nitrogen supplied from a nitrogen-containing source gas. The III/V ratio is adjusted or controlled to create a Group III metal-ric environment within the reactionchamber conducive to preferential column growth. The reactant vapor species are deposited on the growth surface to produce single-crystal MIIIN columns thereon. The columns can be employed as a strain-relieving platform for the growth of continuous, low defect-density, bulk materials. Additionally, the growth conditions can be readjusted to effect columnar epitaxial overgrowth, wherein coalescence of the Group III nitride material occurs at the tops of the columns, thereby forming a substantially continuous layer upon which additional layers can be deposited (J. J. Cuomo, N. M. Williams, A. D. Hanser, E. P. Carlson, and D. T. Thomas, “Method and Apparatus for Producing MIIIN columns and MIIIN materials Grown Thereon,” U.S. Pat. No. 6,692,568).